(1) Field of the Invention
The present invention relates to display devices like a liquid crystal display device or an organic EL display device that use thin film transistors.
(2) Description of the Related Art
A liquid crystal display device has a Thin Film Transistor (herein after TFT) in each of the pixels to control taking data in the pixel. Sometimes a driving circuit is formed by TFTs.
An organic EL display device has a first TFT as a switching element to control taking data in the pixel, and a second TFT that controls a current to the light emitting element. Sometimes a driving circuit is formed by TFTs.
Therefore, current-voltage characteristics of the TFTs have a significant influence to characteristics of the liquid crystal display device and the organic EL display device. In TFT, a current in the drain or in the source is controlled by a voltage Vgs between the gate electrode and the source electrode. By the way, the drain and the source interchange periodically in the TFT, thus, the terms of the source and the drain may be interchangeably used in this specification except specifically noted.
The TFT controls the source-drain current Ids by controlling a current at the channel that is formed under the gate electrode; however, an electric field at the edge of the channel in a plan view tends to be irregular, consequently, the current that flow edge of the channel makes the Ids characteristics irregular.
Patent document 1 (Japanese patent laid open 2000-101084) discloses to suppress a current at the edge of the channel of the TFT by making a resistance at the edge of the channel large, thus, to prevent the Ids characteristics from being irregular.